According to a conventional electric power conversion apparatus, which converts electric power supplied to a rotary electric machine, switches over on/off states of a plurality of switching elements. In case that the rotary electric machine is an electric motor, for example, the motor cannot produce desired torque and will damage mechanical devices such as gears coupled to an output shaft of the motor, if the electric power conversion apparatus has failure. According to JP2006-50707A, therefore, a biasing circuit is provided to detect abnormality in electric power supply cables. The biasing circuit applies a bias voltage to an arbitrary one of the power supply cables.
Since voltages of the power supply cables of all phases are detected, the voltages must be detected at many locations and processing of detecting abnormality is complicated.